a) Field of the invention
The present invention relates to a gradient index lens component which is suited for use as an image pickup lens component in electronic image pickup apparatuses such as video cameras and also to an image pickup apparatus using the gradient index lens component.
b) Description of the prior art
In recent years, now that electronic image pickup apparatuses are widely used in domestic video cameras, television telephones and door phones with cameras, it is an important theme to develop a compact lens system which is usable in these electronic image pickup apparatuses and can be manufactured at a low cost. Each of conventional lens systems which are used in these electronic image pickup apparatuses is generally composed of three to six lens elements, whereas there are known other lens systems each of which is composed of a single lens element for configuring the electronic image pickup apparatuses compactly and manufacturing them at low cost. As a conventional example of the conventional lens systems each composed of a single lens element, Japanese Patent Kokai Publication No. Hei 4-191716 discloses a lens system which uses an aspherical surface. As lens systems which are to be used for different purposes, Japanese Patent Kokai Publication No. Sho 60-91316 discloses a gradient index rod lens element having planar surfaces on both sides which is to be used as a collimator for a laser, Japanese Patent Kokai Publication No. Sho 50-11045 proposes a gradient index rod lens element having planar surfaces on both sides which is to be used as an optical fiber connector and Japanese Patent Kokai Publication No. Sho 60-140307 discloses a lens component composed of a single lens element having a concave surface on the object side and a planar surface on the image side which is to be used as an optical pickup and a collimator for lasers. Further, known as an objective lens system for endoscopes consisting of two lens elements is a lens system disclosed by Japanese Patent Kokai Publication No. Sho 52-29238 which is composed of a combination of a homogenous concave lens element and a gradient index lens element having planar surfaces on both sides.
Out of the conventional examples mentioned above, the lens system which is composed of the three to six lens elements has the drawbacks that it has high manufacturing cost due to the large number of lens elements and allows optical performance to be degraded due to assembling errors. Further, the lens system disclosed by Japanese Patent Kokai Publication No. Hei 4-191716 which is composed of the single lens element has a defect that it produces curvature of field, distortion and chromatic aberration in large amounts. Each of the gradient index lens elements which are disclosed by Japanese Patents Kokai Publication No. Sho 60-91316 and No. Sho 50-11045 which have the planar surfaces on both the sides is configured for picking up monochromatic images has optical performance mainly for axial rays, produces offaxial aberrations in large amounts, and does not take chromatic aberration into consideration. The lens system disclosed by Japanese Patent Kokai Publication No. Sho 60-140307 which is composed of the single gradient index lens element having the concave surface on the object side and the planar surface on the image side produces astigmatism in a large amount and does not take chromatic aberration into consideration. Furthermore, the lens system disclosed by Japanese Patent Kokai Publication No. Sho 52-29238, which is composed of the two lens elements, has drawbacks that it requires high manufacturing cost and that it produces chromatic aberration in a large amount.